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A framework for evaluating forest ecological vulnerability in tropical deforestation fronts from the assessment of forest degradation in a landscape approach: Case studies from Brazil and Vietnam

Bourgoin C.. 2019. Montpellier : AgroParisTech, 233 p.. Thèse de doctorat -- Géographie.

The conservation of tropical forest cover is a key to ensuring sustainable provision of multiple ecosystem services. However, increasing demography, demand for agricultural products and changes in land uses are affecting forest sustainability through degradation processes. A first step to tailor effective forest management is to identify most vulnerable forests and to characterize their drivers. The objective of this thesis is to develop a multidimensional approach to assess forest degradation and the relations with the broader dynamics of land use/cover towards the evaluation of forest ecological vulnerability. The thesis was applied in Paragominas (Brazil) and Di Linh (Vietnam) where large-scale deforestation driven by commercial agriculture shaped the landscape into land use mosaics with increasing degradation pressures. In Paragominas, degradation is linked with selective logging and fire implying changes in forest structure. We estimated the potential of multisource remote sensing to map forest aboveground biomass from large-scale field assessment of carbon stock and investigated the consequences of degradation history on forest structures. We found that canopy textures correlated with forest structure variability and could be used as proxies to characterize degraded forests using very high resolution images. Based on environmental, geographical factors and landscape structure metrics, we demonstrated that 80% of forest degradation was mainly driven by accessibility, geomorphology, fire occurrence and fragmentation. The drivers of degradation acted together and in sequence. The combination of current forest state, landscape dynamics and information on degradation drivers would be at the basis of ecological vulnerability assessment. In Di Linh, degradation is driven by encroachment of coffee-based agriculture. Field inventory of the different forest types and other landscape elements combined with Sentinel-2 images allowed to map with high precision the current land cover. We constructed trajectories of landscape structure dynamics from which we characterized the expansion of the agricultural frontier and highlighted heterogeneous agricultural encroachment on forested areas. We also identified degradation and fragmentation trajectories that affect forest cover at different rates and intensity. Combined, these indicators pinpointed hotspots of forest ecological vulnerability. Through the developed remote sensing approaches and indicators at forest and landscape scales, we provided a holistic diagnosis of forests in human-modified landscapes This thesis aims to pave the way for tailored and prioritized management of degraded forests at the landscape scale.

Mots-clés : Écologie forestière; forêt tropicale humide; vulnérabilité; dégradation des forêts; biomasse aérienne des arbres; couvert; cartographie; télédétection; modèle mathématique; services écosystémiques; brésil; viet nam; amazonie; déforestation; canopée

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